diff: abort scheduled tasks on rediff

viewer.scheduleRediff();
viewer.rediff();

shouldn't cause performing calculations twice

platform/diff-impl/src/com/intellij/diff/tools/util/base/DiffViewerBase.java

@@ -127,6 +127,7 @@ public abstract class DiffViewerBase implements DiffViewer, DataProvider {
   @CalledInAwt
   public final void rediff(boolean trySync) {
     if (isDisposed()) return;
+    abortRediff();
 
     onBeforeRediff();